PLEASANT VALLEY, Mass.—It wasn’t long after dawn—still the cool of the day—but I could feel the weight of summer bearing down. The creek was barely trickling, the beaver pond stagnant. Few insects disturbed its gray surface. Even the birds in this Audubon sanctuary were laying low. I was probably the largest creature walking around these quiet woods, and it would soon be too hot for me to think about anything except reaching the ocean on a vacation still a few weeks in the future.